1. Field of the Invention
The present invention relates to a headphone device having a noise cancelling function, and a signal processing device having a noise cancelling function. Also, the present invention relates to a signal processing method that is suitable for application to a noise cancelling system.
2. Description of the Related Art
In the related art, so-called noise cancelling systems exist and have been put into practical use which are adapted for use in a headphone device and which are configured to actively cancel an external noise that is heard when reproducing the sound of content such as a tune via a headphone device. Broadly speaking, two schemes exist for such noise cancelling systems: a feedback scheme and a feedforward scheme.
For example, Japanese Unexamined Patent Application Publication No. 3-214892 describes the following configuration as a configuration of a noise cancelling system based on the feedback scheme. According to the configuration, a sound signal is generated by inverting the phase of a noise inside a sound tube picked up by a microphone unit that is provided in proximity to an earphone (headphone) unit within the sound tube worn on the ear of a user, and this sound signal is outputted as a sound from the earphone unit, thus reducing an external noise.
Also, Japanese Unexamined Patent Application Publication No. 3-96199 describes, as a configuration of a noise cancelling system based on the feedforward scheme, a configuration in which, basically, a characteristic based on a predetermined transfer function is given to a sound signal obtained by picking up a sound by a microphone attached to the outer casing of a headphone device, and the resulting sound signal is outputted from the headphone device.
When either of the feedforward scheme and the feedback scheme is adopted, the filter characteristic to be set for noise cancelling is set in such a way that noise is cancelled at the position of the user's ear, on the basis of the spatial transfer function for a sound from an external noise source to the position of the user's ear (noise cancellation point), and various transfer functions such as the microphone amplifier/headphone amplifier characteristics.
Under present circumstances, filters for noise cancelling (NC filters) are configured by an analog circuit. In cases where the NC filter is to be configured by an analog circuit, to variably set its filter characteristic for adaptation to different noise environments, for example, a plurality of filter circuits having different filter characteristics are provided, and these filter circuits are switched between each other to effect a change in filter characteristic. However, such a configuration is not practical from the viewpoint of the circuit mounting area or the like. As a result, under present circumstances, it is not possible to change the filter characteristic.
In view of the above-mentioned present circumstances, the present applicant has previously proposed a configuration in which a noise cancelling filter is realized by a digital circuit, as a configuration for variably setting the filter characteristic. That is, the noise cancelling filter is realized by a digital filter using, for example, an FIR (Finite Impulse Response) filter. By adopting a noise canceling system using such a digital filter, a change in filter characteristic can be effected by changing the filter configuration or filter coefficients, and the configuration can be simplified in comparison to the case where the filter is configured by an analog circuit. That is, the configuration for effecting a change in filter characteristic can be achieved in a realistic manner.